This invention relates to a thermal transfer recording medium, more particularly to a thermal transfer recording medium resistant to repeated uses.
In recent years, the techniques in letter printing recording have progressed from the impact recording employing a pressure-sensitive ribbon of the prior art to non-impact recording which enables lowering in printer cost, removal of noise and alleviation in maintenance. Among various non-impact recording methods, the thermal transfer recording method is not only excellent in the above requirements but it is also of interest with respect to the advantages such as stability of the image, reliability, etc.
However, in the thermal transfer recording under the present situation, the ribbon has no resistance to repeated uses, and the greatest problem to be solved is lowering in running cost. Thus, development of a thermal transfer recording medium resistant to repeated uses which can take the place of the one time ribbon has been desired. Some techniques for this purpose have hitherto been disclosed.
For example, Japanese Unexamined Patent Publication No. 68253/1969 discloses a method in which a fine porous layer of a resin component is formed by use of a volatile solvent and the ink is permitted to be oozed out through thermal fusion from the heat-fusible colorant layer (hereinafter referred to merely as the colorant layer) with the use of said resin component as the solid matrix, and Japanese Unexamined Patent Publication No. 105579/1980 discloses a method in which a heat-fusible ink is incorporated into a colorant layer having similarly the heat-resistant microporous reticulate structure, and a polymer with a heat resistant temperature of 120.degree. C. or higher is employed as the solid matrix.
Also, Japanese Unexamined Patent Pubication No. 160691/1982 discloses a method in which an ink employing a solvent dye as a colorant in an inorganic or organic fine powder (e.g. carbon black) as the solid matrix is permitted to be oozed out.
Further, Japanese Unexamined Patent Publication No. 185192/1982 discloses a method in which an ink is impregnated into a porous paper as the solid matrix and permitted to be oozed out through thermal fusion.
Any of these techniques is based on the concept of having an ink oozable by heating existed in a porous solid matrix and being transferred little by little from the colorant layer. Such a concept utilizes, for example, the technique of the carbon paper for pressure-sensitive transfer as disclosed in Japanese patent Publication No. 13426/1960 for the carbon paper for thermal transfer.
However, in the case of thermal transfer, the presence of the solid matrix for transferring little by little the ink can be at the same time another great drawback.
That is, during application of energy, the solid matrix itself will not be fused or mixed, thus being substantially non-transferable. By containing such a non-transferable material within the colorant layer, the colorant layer as a whole is made thicker and the transfer efficiency per unit volume of the colorant layer will be lowered. Consequently, there may be induced lowering in sensitivity, lowering in letter printing speed, lowering in density of the transferred image, excessive load on the thermal head or deterioration of printed letter quality such as resolution, etc. Accordingly, development of a recording medium resistant to repeated uses without use of a solid matrix has been desired, and its basic performance is that the colorant layer and the support must have sufficient adhesive force to satisfy at least the requirement that the whole thickness layer of the colorant layer should not be transferred all at once.
As the techniques for this purpose, pressure-sensitive sheets have been known for a long time, but also in the field of thermal transfer, techniques interposing adhesion layers are disclosed in Japanese Unexamined Patent Publications Nos. 68253/1979, 105579/1980, 36698/1982 and 96992/1984.
However, as described above, when no solid matrix is employed, only interposition of an adhesion layer can hardly control in an adequate manner the amount of ink transferred.
More specifically, according to the investigations by the present inventors, by only combining the basic materials to be generally employed in one time ribbon of the prior art, for example, various waxes, various colorants such as carbon black, pigment, low softening resins, softeners {oily or semi-solid substances (normal temperature) such as castor oil, mineral oil, hydrogenated vegetable oil, white petrolatum, lanolin, hydrophilic petrolatum, etc.} and incorporating them in the colorant layer, the amount of ink transferred cannot adequately be controlled. For example, when the colorant layer is attempted to be softened by using in combination a softening agent which is semi-solid or oily at room temperature and a solid heat-fusible substance, the amount of ink transferred cannot be optimized for various kinds of transfer papers. Also, while it is useful to impart film forming property with a low softening resin similarly as in the case of one time ribbon, or ensure adhesive force with the lower layer, such measures cannot make the recording medium competent enough to give high quality printed letters of high density on any transfer paper and yet make it resistant to repeated uses.
Although for example, even letter printing can be repeated for a large number of times on a paper with high Bekk smoothness, for a paper with low Bekk smoothness, the amount of transfer for the first time may be too much, whereby the density of the printed letter of the second time or thereafter may be lowered.
Alternatively, if it is desired to obtain an appropriate number of letter printing on papers with low smothness, there will ensue the problem such that the amount of transfer is too small for one time and the density is too low in cases of a paper with high smoothness.
Moreover, there are also involved problems, irrespectively of the smoothness of the transfer paper, such that the density is essentially low or that the quality of printed letter is bad.